In a printed wiring board provided with a high-speed integrated circuit, a through current, which is generated at a power supply terminal when the integrated circuit is switching, is one of factors of radiated noise or conducted noise. This is because the generated through current flows via a power wiring to form a long loop which can generate the radiated noise or an excitation source for resonance between a power plane and a ground plane. In addition, in a case in which the integrated circuits share a power source in use, a through current flowing from one circuit may be conducted to another circuit to exert an adverse affect on the latter. Furthermore, in another case in which the power wiring is connected to a connector, noise may spread out through the connector to other boards or may be radiated noise from a cable.
As a countermeasure against the through current generated at the power supply terminal of the integrated circuit, a decoupling capacitor is interposed between a power line and a ground around the power supply terminal so that the through current generated at the power supply terminal reflows into the ground around the power supply terminal, thereby eliminating propagation of RF energy on a power supply plane. Further, in a case in which a plurality of integrated circuits share a power source in use, it is known that the power source used for each of integrated circuits can be decoupled at higher frequencies using a power-decoupling element to avoid the above-described problem. Furthermore, the power-decoupling element is also around the power supply terminal of a connector to avoid noise spreading from the connector.
In recent years, however, integrated circuits have become more complicated and more sophisticated with a very large number of power terminals, and the number of the integrated circuits used on a printed wiring board may be several tens or more. Thus, it is not easy to check whether or not such countermeasure as describe above is adopted correctly in all cases.
Considering the above problem, as to existence and decision of both optimum interposed position and optimum capacitance of a decoupling capacitor interposed around a power supply terminal of an integrated circuit, for example, a checking method and a checking system disclosed in the following patent document 1 is known.
[Patent Document 1] JP-2002-16337-A